1. Field of the Invention
The present invention relates to a legged robot and its control method.
2. Description of Related Art
Various techniques have been proposed for generating a walking trajectory of a legged walking robot. As one technique of generating a walking trajectory, a technique of generating the trajectory of the center of gravity position of a robot is known (e.g. Japanese Unexamined Patent Application Publication Nos. 2004-167676 and 2006-212736). Japanese Unexamined Patent Application Publication No. 2004-167676 discloses a technique of calculating the trajectory of the center of gravity in which a ZMP (zero moment point) that is calculated by a ZMP equation coincides with a target ZMP. In this description, the term “trajectory” refers to data that describes a change in position over time.
Generating the trajectory of the center of gravity of a robot generally requires the trajectory of the angular momentum of rotation about the center of gravity of the robot. However, the related arts focus on obtaining the trajectory of the center of gravity of a robot and take no consideration about a method of setting an appropriate angular momentum while satisfying condition to prevent the falling down of a robot. For example, the related arts assume the angular momentum of a robot to be 0 or treat the angular momentum as a disturbance. If the angular momentum is assumed to be 0, the trajectory is generated on the assumption that the angular momentum is conserved.
Because the falling down of a robot is partly caused by the time rate of change of the angular momentum, generating the trajectory of the center of gravity assuming that the angular momentum in motion is 0 actually results in an unnatural trajectory in which the upper body of a robot rotates.
In the case where a biped locomotion robot performs on-the-spot stepping motion by setting the angular momentum to 0, for example, if the center of gravity shifts to the left, the upper body rotates to the right as shown in FIG. 5B because the angular momentum of 0 is conserved. Specifically, because the robot tries to maintain the angular momentum to be 0 against the movement of the upper body, the upper body rotates so largely as to be off-balance. Likewise, in the case of running motion, the upper body of the robot rotates to the back by every one-step forward, resulting in the backward-bent posture.
Such motion is not only unnatural in appearance but also generates a large torque on a hip joint or the like of the robot, which causes an excessive load on the robot. Further, in the case where human perform the stepping motion, they can implement the motion with the posture maintained without rotating the upper body. Thus, in order for the robot to implement the motion in the same natural posture, it is necessary to set the angular momentum to an appropriate value rather than setting it to 0.
As described above, according to the related arts that treat the angular momentum as 0, the upper body of the robot behaves in an unnatural way.